The old way of making sodas was through a simple fermentation process that creates a probiotic with live yeast. You make juice or an infusion, add yeast to it, let it ferment for a short time in a container, then transfer it to a closed bottle. As fermentation continues within the closed bottle, pressure builds and creates carbonation. The basic principle is extremely simple. All you need is some water, a source of sugar, yeast, and some plants or berries for flavor.